Sex In The Scanner

January 12th 2009

Morten L. Kringelbach D. Phil, is a Senior Research Fellow at the Department of Psychiatry, University of Oxford, and a Professor at Aarhus University Denmark. His most recent book, The Pleasure Center: Trust Your Animal Instincts argues that if we understand and accept how pleasure and desire arise in the complex interaction between the brain’s activity and our own experiences, we can discover what helps us enjoy life, enabling us to make better decisions and, ultimately, lead happier lives. After reading an article on Current.com in which Krigelbach is quoted, I wanted to learn more about the relationship between our brain and sexual pleasure. The fascinating excerpt below looks more deeply at the relationship.

An IgNobel-Prize-winning experiment on sexual intercourse in a whole-body magnetic resonance scanner was published in the Christmas issue of the British Medical Journal. The IgNobel Prize is given to research “that makes people laugh and then think.” The study was carried out in Holland by Willibrord Schultz and colleagues with a total of 13 experimental copulations by eight couples and sexual arousal in three single women. Because these scanners were designed for only one person, club members have to have a certain physiognomy and acrobatic abilities.

Despite the cramped conditions, the nine women reported orgasms, which they, however, rated as superficial. The experiments did not investigate the brain activity in the participants, but only the physiology, which was so difficult for men that they were given potency enhancers.

It was the first time that researchers were able to peer inside the human body during intercourse; they found that in the missionary position, the penis has the shape of a boomerang. A third of its length consists of the root of the penis, so that the average penis including the root was all of 22 centimeters (8.7 inches) in the experiments.

This discovery was in contrast to previous anatomical drawings by Leonardo da Vinci in 1493 of a straight penis during intercourse or the “s”-shape drawn by R.S. Kendall in 1933. It was also found that the size of the female uterus did not appear to change with sexual excitement, which at odds with the original findings by Masters and Johnson who, using a manual method, found between 50 to 100% increases of the uterus 20 minutes after organism. These results were thought to be due to increased blood flow, but it is now more likely to have been due o imprecise measurements.

Brain scanning of sexual excitement and orgasm is remarkably rare. Of course there are technical problems with such studies, such as the ability to keep one’s head still during the experiment. But such problems are not so insurmountable that they can explain why there are so few studies. The sexual instinct would appear to be taboo to a degree that surpasses even drug studies, of which many more exist.

It was not until the end of 2003 that an interesting study of the activity in the male brain during orgasm was published; that was followed by a study of female orgasm in 2006. In 1985, scientists had tried using electroencephalography (EEG) to uncover the effects of male masturbation, but surprisingly there were no significant changes in activity. Another study was published in 1994 by a Finnish group using single photon emission computed tomography (SPECT) that found that orgasm is related to less activity in the whole brain, but more activity in the right prefrontal cortex. Unfortunately, the spatial resolution of SPECT is limited, and it is therefore difficult to evaluate these findings.

The Dutch neurologist Janniko Georgiadis and colleagues used positron emission tomography (PET) scanning to minimize the problems with orgasm-induced movement and found increased blood flow in many parts of the male brain when comparing orgasm with only sexual excitement. The strongest activity was found deep in the brainstem in the ventral tegmentum, which is closely linked to dopamine release. Similar activity has been found in experiments using rewards both natural, such as food, and artificial, such as heroin. Dopamine release appears to be linked to rewarding behavior such as ejaculation, in this instance. Other areas showing increased activity included the mid-anterior orbitofrontal cortex, anterior insula, and cerebellum, which are involved in regulation of emotional state and learning of motor responses. The researchers also found less blood flow in the amygdala and entorhinal cortex in orgasm, with similar responses in the amygdala having been found in studies on cocaine.

The female orgasm has also recently been studied with PED by the same group of researchers. Although male orgasm is directly linked with reproduction through ejaculation, female orgasm does not serve a direct reproductive role. However, it has been proposed to serve a role for sperm retention and for attachment. In the experiment, heterosexual women achieved their orgasms through clitoral stimulation from their male partner, and their level of arousal was measured both by verbal ratings and with a rectal probe that measures rectal pressure variability. Compared to rest, the orgasm was linked to decreased activity in left mid-anterior orbitofrontal cortex, inferior temporal gyrus, and the anterior temporal pole. The results fit well with the proposed role of the orbitofrontal cortex as a mediator for subjective hedonic experience. The women’s level of sexual arousal also was measured and found to correlate with activity in the medial ventral midbrain and the caudate nucleus. So these measures of desire were found to correlate with brain regions that have been implicated in the release of dopamine. Overall, the results would seem to support the distinction between separate brain regions implicated in wanting and liking.

There also have been a couple of studies of sexual excitement related to erotic images. The results are difficult to interpret because they do not include objective measurements of the sexual arousal. One study purports to have shown gender differences in brain activity when the participants were shown identical erotic images. Specifically, more activity was found in the amygdala and hypothalamus in men than in women. It is difficult to interpret such results because they can have many competing causes related specifically to the experimental setup and methods. More generally, it is also doubtful to what extent it is possible to distinguish between male and female brains…